A novel optical velocimeter has been designed in order to measure with a high resolution electrophoretic mobility distributions of suspended particles. It uses a transmission grating on which the dark‐field image of the particles is focused. A spectral analysis of the transmitted time‐modulated light intensity allows the determination of the velocity spectrum. The principle, a description of the apparatus, and some results are given. The velocity resolution is significantly better than with the laser Doppler techniques for different reasons which are discussed.